1-3 months, usually in late summer or early fall. You can estimate how far along a bird is in molt. They molt one feather tract at a time, starting with the head and ending with the abdomen. The primaries molt in a certain order (innermost primary first, followed by second innermost, third innermost...)

My older hens all molted at the same time this year-early fall. It was very obvious, bare patches on their wings, necks, etc. They stopped laying completely, and their combs went from red to pink. They became more docile and seemed to really go down in the pecking order, letting the pullets eat first, etc. I don't use any kind of artificial light in the coop and they are just starting to lay again. I'm ready for that bumper crop of eggs again, I've been eating pullet eggs all winter